Friends of the Pleistocene Seminar Series
The Zealandia Switch – Hypothesising the Southern Hemisphere in the Driving Seat of Global Climate, with David Barrell, GNS Science, Dunedin
Rhythmic alignment of Quaternary-age glacial cycles and Earth’s orbital variation pattern points to orbital control of climate, but the operational mechanisms remain under discussion. Despite opposing orbital forcing parameters in the Northern and Southern hemispheres, proxy paleoclimate data convincingly demonstrate global synchrony of glacial-cycle climate shifts. The Milankovitch model invokes orbitally-controlled extents of Northern Hemisphere continental ice sheets via summer solar radiation intensity, with the climatic signature synchronized globally through oceanic/atmospheric processes. An alternative view emphasizes orbital controls affecting the highly energetic atmospheric and oceanic circulation systems of the Southern Hemisphere. The Zealandia Switch hypothesises central roles in glacial-interglacial climate shifts for the Southern Hemisphere mid-latitude westerlies and the Australia/Zealandia continental platforms (Denton et al. 2021).
Often overlooked is that ice-age millennial-scale climate patterns, as registered in Greenland ice cores, had global uniformity ~50% of the time, interspersed with climatic episodes apparently anti-phased between the hemispheres and described as a bipolar seesaw. Denton et al. (2022) hypothesize that the apparently anti-phased episodes occurred under globally synchronous climatic conditions, with each episode initiated by a bi-hemispheric shift to warmer-than-usual summers. Dubbed ‘Heinrich’ summers, enhanced seasonal melt of northern continental ice sheets formed meltwater floods into the North Atlantic Ocean. This resulted in unusually extensive winter sea ice that created an extreme seasonality in adjacent regions, with mild summers but ultra-cold winters. A southern driver is suggested for orbital-scale as well as millennial-scale ice-age climate shifts.
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